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Lizard ‘racing stripes’ redirect predator attacks

When you take a look at a five-lined skink, its tail is probably the first thing to grab your eye. If so, you wouldn’t be alone: Predators are also attracted to its bright blue color, and they tend to attack these bright tails more than other body parts. That’s a good thing for the skink, because lizards can drop their tails when in danger and regrow them later. Now, new research shows that the lines running along the body of the five-lined skink—and similar color patterns in lizards around the world—may also redirect predator attacks by making the lizards appear slower than they actually are.

Scientists have long thought that bright stripes in nature may function as optical illusions, helping animals avoid attacks by messing up how predators perceive motion. The explanation is one of many that researchers have used to explain the stripes of zebras. And it has even made its way into the military: Allied ships in World War I were painted with disorienting black-and-white stripes in an attempt to avoid torpedo strikes. But the evidence for whether this so-called “motion dazzle” works—in nature or in war—has been mixed.

So Gopal Murali, an evolutionary biologist at the Indian Institute of Science Education and Research, Thiruvananthapuram, decided to look at how such stripes might function in lizards, many of which sport racing stripes similar to the five-lined skink. But instead of traipsing into the woods to find animals that prey on lizards, he and his colleague turned to another kind of predator: graduate students.

“Humans are fantastic because you can tell them what to do and you can get a lot of them very easily,” says Anna Hughes, a visual ecologist at the University of Cambridge in the United Kingdom, who wasn’t involved with the study.

Using a touch screen computer game that rendered lizards as patterned rectangles zipping across a screen, the team asked the students to attack (or, in this case, click) the front half, which was covered in either stripes or blotches. When the front half was striped, they were 25% less likely to hit their target. That difference disappeared when the stripes were on the back half instead, the team reports today in Royal Society Open Science.

To explain the phenomenon, the authors created a second game in which participants were asked to compare the speeds of two virtual lizards—one with stripes and one with blotches. If subjects said that one lizard was faster than the other, the game started over, with the “faster” lizard slowed down ever so slightly. That process repeated until subjects thought the two lizards were moving at the same speed. In reality, they weren’t: By the end of the experiment, the striped lizards were moving 5% faster on average. That shows that lizards with striped patterns “are perceived to move slower,” Murali says.

Together, the two games show how these stripes might work in nature. If a bird goes in for the kill but underestimates a skink’s speed, it will come up with nothing but a mouthful of disembodied tail—and the skink will live another day.

“They took a really interesting, innovative approach to asking a question that … a lot of people who are interested in lizard ecology have,” says Christian Cox, an evolutionary biologist at Georgia Southern University in Statesboro. And now that there’s some solid evidence for why so many lizards have these stripes, it opens up all kinds of new questions, he says, along with providing a basis for future work with more natural predators.

The study may also provide some direction for researchers who study “motion dazzle” using computer simulations. Such tests often just compare striped objects and nonstriped objects without a specific organism in mind, Hughes says. A focus on the specific patterns and lifestyles of one group of animals “is something the field has really been missing”—which may account for the mixed results the field has found so far. It may be that researchers were trying to find simple explanations for phenomena that, in nature, have lots of different functions in different animal groups.

Murali himself has big plans for exploring the idea further. One of his ongoing studies looks at the role body size might play in the illusion. Another study looking at lizard family trees will test whether these patterns might play a role in evolution on a larger scale: By giving lizards protection as they move around, stripes may allow them to explore different habitats, increasing the speed at which new species appear.